System for improving the quality of a received radio signal

ABSTRACT

A delay spread is created in a digital radio signal to reduce the coherence bandwidth and facilitate frequency hopping to reduce the effect of fading losses within an enclosed propagation environment The delay spread is introduced into the signal in several ways. One technique disclosed employs a transmitter with two separate antennas one of which transmits the digital signal and the other of which transmits the same signal after a phase delay has been introduced into the signal. The carrier frequency of the signals is hopped between at least two frequencies and the receiver processes the resulting signals. In another embodiment, a single transmit antenna is used but the signal is received by two different antennas with the output signal from one of those antennas being phase delayed before combining it with the other prior to processing by the receiver circuitry. Phase delay is also introduced at baseband into the signals to be transmitted by rotating the I and Q components of the waveforms before modulation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to radio systems and, more particularly, toimproving the efficiency of frequency hopping as a technique of reducingthe effects of fading in an enclosed region of signal propagation.

2. History of the Related Art

Radio Transmission Problems

The quality of the signal received by a mobile station from a basestation is affected from time to time by natural phenomena inherent inthe use of radio signals to communicate. A factor common to most of theproblems related to radio reception is that the desired signal at thereceiver is too weak, either in comparison to thermal noise or incomparison to an interfering signal. An interfering signal can becharacterized as any undesired signal received on the same channel bythe receiver as the desired signal.

In the case of cellular radio systems where all of the frequencies inthe available bandwidth are being reused throughout the cellular grid,the efficiency of the radio system is generally limited by the amount ofinterfering radio signals received rather than thermal noise.

One phenomenon which occurs to limit the quality of a received signalwithin a radio system is path loss. Even when there are no obstaclesbetween the transmitting antenna and a receiving antenna, the receivedsignal becomes progressively weaker due to the increasing distancebetween the base station and the mobile station. The received signalpower is inversely proportional to a value somewhere between the squareand the fourth power of the distance between the transmitting andreceiving antennas.

A more common transmission problem in mobile radio systems used in anenvironment where there are objects such as buildings present, is thatof log-normal fading. This phenomenon occurs as a result of theshadowing effect produced by buildings and natural obstacles such ashills located between the transmitting and receiving antennas of amobile station and a base station. As the mobile station moves aroundwithin the environment, the received signal strength increases anddecreases as a function of the type of obstacles which are at thatmoment between the transmitting and receiving antennas. The term"log-normal," comes from the fact that the logarithm of the receivedsignal strength takes the form of a normal distribution about some meanvalue the minimum values of which are referred to as fading dips and thedistance between which may be on the order of 30 to 60 feet.

A third phenomenon which effects signal strength within a mobile systemoperated in an urban environment is that of Rayleigh fading. This typeof signal degradation occurs when the broadcast signal takes more thanone path from the transmitting antenna to the receiving antenna so thatthe receiving antenna of the mobile station receives not just one signalbut several. One of these multiple signals may come directly from thereceiving antenna but several others are first reflected from buildingsand other obstructions before reaching the receiving antenna and, thus,are delayed slightly in phase from one another. The reception of severalversions of the same signal shifted in phase from one another results inthe vector sum of the signals being the resultant composite signalactually received at the receiving antenna. In some cases the vector sumof the receive signal may be very low, even close to zero, resulting ina fading dip wherein the received signal virtually disappears. In thecase of a moving mobile station, the time that elapses between twosuccessive fading dips due to Rayleigh fading depends upon both thefrequency of the received signal and the speed at which the mobile ismoving. The distance between two fading dips due to Rayleigh fading maybe on the order of 7 inches for the 900 megahertz radio band.

Referring to FIGS. 1A and 1B, there is illustrated a perspective modelof the frequency/distance received signal fading pattern within atypical mobile radio operating environment. FIG. 1A represents thereceived signal field of a radio signal operating within an urban areaat a frequency of 100 megahertz while FIG. 1B represents a radio signaloperating in an urban area with a signal frequency of 300 megahertz. Itcan be seen from these diagrams how the strength of the signals vary,creating periodic fading dips which are both distance and frequencydependent.

In the case of digital radio systems, such as those in which timedivision multiple access (TDMA) modulation is used, other radiotransmission difficulties arise. One of these difficulties, referred toas time dispersion, occurs when a signal representing certain digitalinformation is interfered with at the receiving antenna by a different,consecutively transmitted symbol due to reflections of the originalsignal from an object far away from the receiving antenna. It thusbecomes difficult for the receiver to decide which actual symbol isbeing detected at the present moment. Another transmission phenomenoninherent in the use of TDMA modulation is due to the fact that eachmobile station must only transmit during a particular allocated timeslot of the TDMA frame and remain silent during the other times.Otherwise, the mobile will interfere with calls from other mobiles towhich are assigned in different time slots of the same frame.

Conventional Solutions to Radio Transmission Problems

There have evolved a series of techniques which are used to combat thesignal degenerative phenomenons which occur in radio transmissionsystems. One solution which is employed to combat the problems of fadingof a digital radio signal, from both log-normal fading and Rayleighfading, is that of coding and interleaving. This is a technique in whichthe information representing various items of digital information isorganized into blocks, and consecutive ones of a series of blocks, forexample, four bits each, are organized into frames. If each of theconsecutive bits of information are sent in the same order as they aregenerated by the speech encoder, the occurrence of a fading dip wouldtotally obliterate several consecutive bits of information which wouldthus be lost from the communication stream and result in a gap in thespeech to be recreated from them. With the technique of interleaving,however, the consecutive bits of information are systematicallyseparated from one another and rearranged in a transmission stream inwhich they are, rather than contiguous to one another, separated in timefrom one another with each one forming one bit of a separate block ofinformation. At the other end of the transmission stream the rearrangedbits are removed from the blocks of data in which they were transmittedand reconstructed to again be contiguous to one another. When each ofthese bits representing speech data are separated from the other bits towhich they are normally contiguous in time and "interleaved" among otherbits not normally contiguous to one another in time and then an entireblock of bits is lost from the transmission stream during a fading dip,at least some portion of that lost block can be constructed from thebits which were not lost during the dip because they were interleavedinto other blocks which were not lost due to fading. In the case of amoving mobile station, a fading dip only occurs for a very brief periodof time as the mobile passes through the region of fading and back intoan area of good reception.

One technique used to secure a digital radio transmission againstinterference is that of error correction coding in which the bits ofinformation to be transmitted are encoded with a correction code so thatif bits are lost during transmission they can be recreated with arelatively high degree of accuracy at the receiver site by the errorcorrection code circuitry. A part of the procedure used in correctioncoding of a digital signal transmission stream is that of interleaving.

An assumption inherent in the use of interleaving techniques with errorcorrection coding is that the mobile is moving so that it passes througha fading dip relatively quickly and only experiences loss of arelatively small block of the digital information due to attenuationwhile it is located in the region of the fading dip. In the case oftransmission environments which are indoors, for example, in aconvention center or office building, a mobile station is relativelyslow moving or perhaps even stationary. Thus, if the mobile happens tobe in a physical location which is subject to a fading dip, it does notpass through that dip quickly and thus a large amount of information islost due to the fade. Losses of large blocks of information cannot becorrected by mere interleaving and error correction coding.

Another technique used to compensate for transmission difficulties in aradio system is that of frequency hopping. In the use of frequencyhopping the radio transmission and reception are at one carrierfrequency for one instant of time and then a very short time later thetransmission and reception is "hopped" to a different frequency. When atransmitted radio signal is at a different carrier frequency, it is notsubject to the same fading pattern because such patterns are frequencydependent and thereby different for different frequencies. Thus, astationary mobile station which may be in the trough of a fading dip atone carrier frequency might get relatively good reception at a differentfrequency. In this way, frequency hopping is used to further limit theamount of signal loss to a relatively short segment of the actualtransmission time span and thus allow the signal processing circuitry tocompensate for the loss of broadcast information with interleaving anderror correction coding by reconstructing the lost portions of thetransmission.

One important aspect of frequency hopping is that the two or morecarrier frequencies between which the signal is successively hopped musteach be separated by a certain minimum amount in order to experienceindependent fading on the different frequencies. In other words, thefrequencies between which the signals are hopped must be sufficientlydifferent from one another so that if the received signal is in a fadingdip on one frequency it should not be in a fading dip on the otherfrequency. If the two frequencies are very close together it is morelikely that the received signal will be weak due to fading at bothfrequencies. If, however, the two frequencies are separated from oneanother by a sufficiently large value then it is less likely that thereceived signal will be in fading dip on both frequencies. Theseparation between the two hopping frequencies which is sufficient toobtain independent fading one from the other is called the coherencebandwidth. If each of the two carrier frequencies used for hopping arewithin the coherence bandwidth, the signals received on each of the twofrequencies will be highly correlated. If the carrier frequencies areseparated by more than the coherence bandwidth then it is likely thatthe signals that are received on each of the two frequencies will beuncorrelated and thus, will not be in a fade condition at the same time.If the two frequencies are uncorrelated from one another the radioreceiver will probably not experience a fade in the other frequency whenone of them is in the fade condition.

When frequency hopping is being used in a TDMA radio signal, and thesignal is being hopped across a different frequency during each of theseveral successive TDMA time slots, with each of the different carrierfrequencies being reasonably far away from one another, then it is verylikely that the signals received in each of the time slots arecompletely uncorrelated from one another. Moreover, if error correctioncoding and interleaving are used across the successive slots in whichthe signal is received, at least half of the bits received during twosuccessive time slots will not be subjected to a fade condition, inwhich case the error correction coding and interleaving will do asatisfactory job of reconstructing the complete signal content despitethe loss of content from one of the two slots.

In considering coherence bandwidth and correlation factors for frequencyhopping applications, the coherence bandwidth is inversely proportionalto the time delay spread of the transmission channel. Time delay spreadoccurs because of multipath propagation. The time difference is betweenthe earliest and latest multipath signals, the main line-of-sight signaland the same signal delayed because one or more reflections creates atime span encompassing both the main signal and its principal echoeswhich contain most of the signal energy.

The coherence bandwidth of a signal is inversely proportional to thetime delay spread of the signal. In the case of indoor implemented radiochannels, the time delay spread is extremely small, for example, on theorder of 50 to 100 nanoseconds. Thus, the coherence bandwidth for suchsignals would be very large, for example, on the order of 10 megahertz.Thus, in order to get independent fading in a frequency hoppingenvironment the frequency difference between two carrier signals wouldhave to be on the order of 10 megahertz, far greater than the bandwidthof most practical systems. It is an even greater problem to hop thecarrier frequency of the signal over three or four separate frequenciesrequiring 30 to 40 megahertz of bandwidth.

One prior approach to the coherence bandwidth problem has been to createsignificant echoes of the broadcast signal by using a secondtransmitter. By delaying transmission on the second transmitter on theorder of a symbol period, the second signal fades independently of thefirst as a function of the spacing of the second antenna from the first.While the receiver can resolve these two signals a more complex receiveris required in order to deal with the signal echoes. The TDMA signalrequires an equalizer while CDMA signals require a Rake combiner.

Another solution has been to use no delay between two separatetransmitting antennas, but, rather, to vary the phase differencesbetween them. This causes the fading at the receiver to vary with time.However, due to receiver mobility, the combined effect can cause thefading to change so fast over time that the receiver processing circuitycannot deal with it. For example, if the receiver has a channel trackerfor coherent demodulations it may not function properly if the fadingchanges too quickly over time. Further, if the mobile is designed to bestationary then a time varying phase may not be tracked by the receiver.

There is thus a need for some mechanism to allow the implementation offrequency hopping over a limited bandwidth to correct for flat fadingchannels in environments such as radio systems implemented in an indoorenvironment.

SUMMARY OF THE INVENTION

The system of the present invention solves some of the foregoingproblems by causing the fading to be different on carrier frequencieswithout the creation of significant signal echoes or significant timevariations in the signal. In one embodiment of the invention, multipletransmit antennas broadcast the same signal with different phases. Inanother embodiment, the signal received on multiple receive antennas arecombined after changing the phase of one or both. In each case, thephase difference is changed as a function of carrier frequency. Thesephase changes can be implemented by using fixed delays between antennasor by using a phase shifter which does not change during a burst butdoes change between bursts.

In another aspect, the system of the present invention includes thecreation of delay spread in a digital radio signal in order to decreasethe coherence bandwidth of a signal so that frequency hopping may beimplemented to correct for fading loss in an environment in which themobile is relatively slow moving, for example, in an indoor environment.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and system of the presentinvention may be obtained by reference to the following DetailedDescription of the preferred embodiment(s) that follow, taken inconjunction with the accompanying drawings, wherein:

FIGS. 1A and 1B are diagrams illustrating the fading loss within amobile radio environment as a function of distance over two differentfrequencies;

FIG. 2 is a block diagram of one embodiment of a system constructed inaccordance with the present invention;

FIG. 3 is another embodiment of a system constructed in accordance withthe teachings of the present invention;

FIG. 4 is a different embodiment of a system constructed in accordancewith the present invention; and

FIG. 5 is a block diagram illustrating the introduction of phase delaysinto a baseband signal by rotation of the I and Q waveforms prior tomodulation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the implementation of the system and method of the present inventiona phase change is introduced into one of two identical radio signalpaths and the degree of change is a function of the carrier frequency ofthe signal. One exemplary approach is to use a fixed delay so that thephase change is equal to the carrier frequency (H_(z)) times the delaygiving rise to a frequency dependant phase relationship between twodifferent transmit antennas. This delay can be implemented, for example,with a surface acoustic wave (SAW) device. In contrast to prior artsystems the delay introduced is very small so that no resolvable echoesof the signal are generated. The delay is only enough to cause a phasechange that is different for different hop frequencies.

More specifically, in the creation of an "artificial delay spread" in asignal being broadcast in a limited bandwidth environment in which themobile is relatively slow moving or stationary, the system of thepresent invention employs two separate antennas. The signal to each ofthe two antennas is delayed relative to the other so that the receivedsignal appears to include both a main signal and a secondary signal. Theextent of the phase difference between the two received signals ischanged as a function of carrier frequency for hopped signals.

In the environment of the invention shown in FIG. 2, a transmitter 21broadcasts a signal through an output lead 22 which is split into twopaths 23 and 24. The signal traveling along path 23 passes through aphase modifier 25 to a broadcast antenna 26. A transmit filter 25A canbe used, for example, in order to introduce a selected phase delay whichvaries with frequency. The other signal passes along line 24 to adifferent antenna 27. The phase modification is introduced to one of thesignals to be on the order of a fraction of the symbol rate so that aminor delay spread is simulated between the respective signals broadcastfrom antennas 26 and 27. A receiver 31 receives the two phase shiftedsignals from antennas 26 and 27 at a single receive antenna 32. At thereceive antenna 32 it appears as if a small amount of time delay spreadhas occurred, so that an equalizer or a Rake combiner is not necessary.Instead, the two signals either constructively add or cancel, dependingon the position of the receiver and the carrier frequency. A frequencyhopping mechanism 21A in transmitter 21 hops the signal broadcastbetween frequencies and another frequency hopping mechanism 31A inreceiver 31 hops the signal reception between frequencies, e.g., from asignal having a first frequency to a signal having a second frequency.

Referring next to FIG. 3, there is shown an embodiment of the inventionin which a signal transmitter 41 broadcasts over a single transmittingantenna 42. That signal is received by both a first antenna 43 and asecond antenna 44. The output signal from the first antenna passesthrough a phase modifier 45 and thence into a combiner 46 in which it iscombined with the unmodified signal from antenna 44. The combined signalis then introduced into the receiver 47 which processes the signals. Thephase modified signals from antenna 44 and the unmodified signal fromantenna 44 are combined in combiner 46 to simulate a delay spreadbetween the two received signals which can be processed in the receiver47. This simulated delay spread produces a smaller coherence bandwidth,which permits effective frequency hopping over a relatively modestbandwidth to combat fading of the signal in an enclosed environment. Asdiscussed in connection with FIG. 2, frequency hopping mechanisms 41Aand 47A in transmitter 41 and receiver 47, respectively, controlfrequency hopping.

The embodiment of the invention shown on FIG. 4 is similar to that ofFIG. 3 in which a signal transmitter 41 connected to a transmittingantenna 42 transmits a signal which is received at a first receivingantenna 43 and a second receiving antenna 44. The signals are introducedinto a receiver 47 and one of the two signals is processed independentlyof the other to introduce a delay 45 (transmit filter 45A) and againsimulate delay spread. This reduces the coherence bandwidth of the twosignals allowing frequency hopping over a reasonable frequency bandwidthto correct for certain fading losses. As discussed above, frequencyhopping mechanisms 41A and 47A control frequency hopping.

The system of the present invention may be implemented in other ways.For example, instead of using a delay, some type of phase offset thatvaries from frequency hop to frequency hop could be employed. In atransmitter such a delay could be introduced at baseband by rotating theI and Q waveforms prior to modulation as illustrated in FIG. 5. Rotationby increments of 0, 90, 180 and 270° are preferable so that the rotatedsignals, I and Q are related to the original signals in the followingsimple ways:

    I'=I; Q'=Q(0°).

    I'=-Q; Q'=I(90°).

    I'=-I; Q'=-Q(180°).

    I'=Q; Q'=-I(270°).

Which degree of rotation could be selected at random from hop to hop orbe a function of the hop frequency control signal or follow some regularfixed pattern.

A similar technique can be used when there are two received signals. Forexample, the signals can be simply added together (0°), or thedifference of the two signals can be taken as well as other means ofmodifying the signals. In U.S. patent application Ser. No. 07/585,910,now U.S. Pat. No. 5,361,404, entitled "Diversity Receiving System", inthe name of Paul W. Dent and assigned to the assignee of the presentinvention, selective diversity is used to select the best combinationwithin a receiving system. However, in the present invention the actualmost desirable combination does not matter. It is only the changing ofthe combination with successive frequency hops in either a random or aknown way which enables the channel coding and interleaving to eliminatelosses due to fading. Somewhat less complex circuity is required toperform these functions in the present invention than in the selectivediversity optimization system of the above-referenced Dent application.

In the embodiment of the present invention which employs multiplereceiver antennas it is possible that the signal delay chosen can be onthe order of a symbol period. In such case if the demodulator can handleecho signals, then a diversity advantage can be obtained without theneed of frequency hopping. While it is difficult to delay one of theantenna signals by as much as a symbol period, this can be accomplishedthrough receiver processing using filters with different group delaycharacteristics.

It should also be noted that while the above invention is described forradio systems, it also applicable to other wireless communicationssystems. Thus, as described above, antennas may refer to any device thattransfers the signal either from the transmitter to a transmissionmedium or from the transmission medium to the receiver. Also, whilefrequency hopping occurs, the multiple access approach within a hop canbe FDMA, TDMA, or CDMA.

Although a preferred embodiment of the method and apparatus of thepresent invention has been illustrated in the accompanying drawings anddescribed in the foregoing detailed description, it is to be understoodthat the invention is not limited to the embodiment(s) disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

What is claimed is:
 1. A method of enabling frequency hopping between asignal of a first frequency and a signal of a second frequency, whichmethod comprises:transmitting the signal from a first transmittingantenna; transmitting the same signal from a second transmitting antennaafter delaying the signal by a preselected time period; processing in areceiver the received signals from the first and second transmittingantennas; hopping the frequency of transmission from the first frequencyto the second frequency; and changing the time period of the delay atbaseband as a function of the frequency of the signal to be transmitted.2. A method of enabling frequency hopping between a signal of a firstfrequency and a signal of a second frequency within a relatively narrowsystem bandwidth as set forth in claim 1, wherein:the signal transmittedis a TDMA signal.
 3. A method of enabling frequency hopping between asignal of a first frequency and a signal of a second frequency, as setforth in claim 1 wherein the delay at baseband between the signaltransmitted from the first and second transmitting antennas is smallrelative to a symbol period of the digital communication signal.
 4. Asystem for enabling frequency hopping between a signal of a firstfrequency and a signal of a second frequency, comprising:means fortransmitting the signal from a first transmitting antenna; means fortransmitting the same signal from a second transmitting antenna afterdelaying the signal by a preselected time period; means for processingin a receiver the received signals from the first and secondtransmitting antennas; means for hopping the frequency of transmissionfrom the first frequency to the second frequency; and means for changingthe time period of the delay at baseband as a function of the frequencyof the signal to be transmitted.
 5. A system for enabling frequencyhopping between a signal of a first frequency and a signal of a secondfrequency, as set forth in claim 4, wherein:the signal transmitted is aTDMA signal.
 6. A system for enabling frequency hopping between a signalof a first frequency and a signal of a second frequency as set forth inclaim 4 wherein the delay at baseband used between the signaltransmitted from the different antennas is small relative to a symbolperiod of the signal.
 7. In a slow frequency hopped digitalcommunication system, a transmitter comprising:means for generating aslow frequency hopped digital communication signal; and means fortransmitting said signal on a plurality of different antennas with eachsignal being transmitted from each antenna separated from the othersignals by phase differences that change at baseband as a function ofhop frequency.
 8. A slow frequency hopped digital communication systemas set forth in claim 7 wherein the phase change between the signals tobe broadcast from the different antennas is made at baseband and thesignal to be broadcast is passed through a plurality of separatetransmit chains.
 9. A slow frequency hopped digital communication systemas set forth in claim 8 wherein said phase delay is introduced into thesignal to be transmitted as a phase offset by rotation of the I and/or Qwaveforms of the original signal prior to modulation.
 10. A slowfrequency hopped digital communication system as set forth in claim 7wherein said phase delay is introduced into the signal to be transmittedby means of a surface acoustic wave (SAW) device.
 11. In a slowfrequency hopped digital communication system, the method whichcomprises:generating a slow frequency hopped digital communicationsignal; and transmitting said signal on a plurality of differentantennas with each signal being transmitted from each antenna separatedfrom the other signals by phase offsets that change at baseband as afunction of hop frequency.
 12. In a slow frequency hopped digitalcommunication system the method set forth in claim 11 wherein the phasedifferences used between the signal transmitted from the differentantennas are small relatively to a symbol period of the digitalcommunication signal.
 13. In a slow frequency hopped digitalcommunication system the method set forth in claim 11 which includes theadditional steps of:making the phase offsets between the signals to bebroadcast from the different antennas at baseband; and passing thesignal to be broadcast through a plurality of separate transmit chains.14. In a slow frequency hopped digital communication system the methodset forth in claim 13 wherein said phase offsets are introduced into thesignal to be transmitted by rotation of the I and/or Q waveforms of thesignal prior to modulation.
 15. In a slow frequency hopped digitalcommunication system the method set forth in claim 11 wherein said phaseoffsets are introduced into the signal to be transmitted by means of asurface acoustic wave (SAW) device.
 16. A method of enabling frequencyhopping between a signal of a first frequency and a signal at basebandof a second frequency, which method comprises:transmitting the signalfrom a first transmitting antenna; transmitting the same signal from asecond transmitting antenna after delaying at baseband the signal by apreselected time period, said delay between the signal transmitted fromthe first and second transmitting antennas being small relative to asymbol period of the signal; processing in a receiver the receivedsignals from the first and second transmitting antennas; and hopping thefrequency of transmission from the first frequency to the secondfrequency.
 17. The method of enabling frequency hopping between a signalof a first frequency and a signal of a second frequency, as set forth inclaim 16, wherein:the signal transmitted is a TDMA signal.
 18. Themethod of enabling frequency hopping between a signal of a firstfrequency and a signal of a second frequency, as set forth in claim 16and which also includes the additional step of:changing the time periodof the delay at baseband as a function of the frequency of the signal tobe transmitted.
 19. A system for enabling frequency hopping between asignal of a first frequency and a signal of a second frequency,comprising:means for transmitting the signal from a first transmittingantenna; means for transmitting the same signal from a secondtransmitting antenna after delaying the signal at baseband by apreselected time period, said delay between the signal transmitted fromthe first and second transmitting antennas being small relative to asymbol period of the signal; means for processing in a receiver thereceived signals from the first and second transmitting antennas; andmeans for hopping the frequency of transmission from the first frequencyto the second frequency.
 20. The system for enabling frequency hoppingbetween a signal of a first frequency and a signal of a secondfrequency, as set forth in claim 19, wherein:the signal transmitted is aTDMA signal.
 21. The system for enabling frequency hopping between asignal of a first frequency and a signal of a second frequency, as setforth in claim 19 and which also includes:means for changing the timeperiod of the delay at baseband as a function of the frequency of thesignal to be transmitted.